Diode pumped lasers involve semiconductor diodes which lase when excited by electrical current. The lasing portions of the semiconductors (usually PN junctions) are positioned near a laser medium (such as a crystal) so that laser energy from the semiconductor diodes is directed into the medium, either directly or via lenses When "pumped" by the laser energy from the diodes, the energy excitation levels (sometimes referred to as population inversions) build up within the atomic structure of the medium and cause the medium to lase. The medium, which is usually elongated and provided with a mirror or a reflective coating at each end, focuses the laser energy along the main axis of the medium. The laser beam exits through an opening in one of the mirrors, or through a portion of one of the mirrors that is partially transparent.
The lasing diode(s) used to excite or "pump" a laser medium can be mounted at one end of the medium, to create an end-pumped laser, or along the side of the medium, to create a side-pumped laser. End-pumped lasers are limited in power since only one diode or a small number of diodes can be mounted in close proximity to the end of a medium. Side-pumped lasers can be more powerful, since many diodes can be mounted along the length of the medium. For more information on diode-pumped lasers, see, e g., W. Koechner, Solid State Laser Engineering (Springer-Verlag, New York, 1988) and the article by G. T. Forrest in Laser Focus Electro-Optics, Nov. 1987, pp. 62-74.
Laser diodes generate substantial amounts of heat and therefore must be cooled if they are to produce substantial outputs If not properly cooled, high temperatures (i.e., increased vibrations of the atomic lattices in the semiconductor material) can interfere with current flow through the semiconductor material, and can damage or destroy the diodes. The laser medium is also subjected to high heat and must be cooled, for comparable reasons High temperatures can also warp the mounting devices, causing the diodes and medium to become misaligned.
The magnitude and importance of the cooling problem can be seen in perspective by considering the efficiencies of diode-pumped lasers. Efficiency is measured by dividing the amount of power carried in the laser beam (expressed in units or watts) by the total wattage consumed by the laser equipment. For a typical side-pumped laser to generate a laser beam carrying one watt of energy, it must dissipate as much as 100 watts of input energy, most of which must be dissipated as heat. Many lasers which cannot otherwise cope adequately with the problem of cooling must be operated only in a pulsed mode; i.e., their output is limited to short bursts of laser energy. Between pulses, such lasers must be deactivated so they can cool. However, it is often desirable to operate lasers in the continuous wave (CW) mode.
Various devices have been developed in the prior art for dissipating the heat generated by diode-pumped lasers. Conventional heat sinks which serve an entire laser head (the assembly which includes the active lasing elements and the mounting components for those active elements) are shown in FIGS. 1 and 4 of U.S. Pat. No. 4,805,177 (Martin et al 1989; assigned to the Applicant's assignee, Laser Diode, Inc.). Such heat sinks usually have fins to increase their surface area. They can be positioned near fans that blow air across the fins, or cooled by pumping cooling water or other liquids across their surfaces or through channels that pass through the heat sinks. Such heat sinks are referred to herein as "system heat sinks," since they serve an entire laser head system. That term distinguishes them from diode heat sinks or medium heat sinks, which interact directly with only the indicated components. Typically, any diode heat sinks and medium heat sinks in a laser head are thermally coupled to a single large system heat sink.
A diode configuration which offers improved heat control is shown in U.S. Pat. No. 4,864,584 which is also assigned to applicant's assignee. That patent discloses a structure with arrays of numerous very small diodes which are closely packed together. Briefly, a semiconductor wafer having a diameter of about 5 cm is generated with hundreds of parallel "laser stripes" consisting of small lines where the doping of the semiconductor material is altered. The laser stripes are arranged in arrays or clusters, such as clusters of four stripes separated by small gaps having no stripes. The wafer is coated with a layer of gold, then it is cleaved by nicking the crystalline material in the gaps between each cluster of four stripes and breaking the crystal along the lines of the nicked material. This creates elongated pieces of material, each piece having four laser stripes running lengthwise. Each piece is then nicked a second time in a direction perpendicular to the laser stripes, then it is broken to form numerous small chips, each chip being roughly one millimeter wide and having four laser stripes that are electrically coupled to the layer of gold. Each chip is referred to as a "laser diode array." A single conductive lead is soldered to the layer of gold on each laser diode array. When the laser diode array is excited by applying a voltage thereacross and passing an electrical current through it, each of the four laser stripes functions as a laser diode pump source.
Each laser diode array is mounted on a diode heat sink, which comprises a small piece of copper or other suitable heat-conducting metal. Numerous diode arrays and their heat sinks are placed next to each other and mounted on a larger diode mount, also made of heat-conductive metal. In that arrangement, the heat generated by the numerous small diodes is distributed more evenly than the heat generated by the larger diodes used in the prior art. Since the heat is distributed more evenly across the diode mount, hot spots (which can adversely effect operation) are minimized.
U.S. Pat. No. 4,864,584 also discloses the use of two rows of diode array pump sources, arranged in opposed relation, one row on each side of a laser medium (see FIGS. 4, 9, and 10 of that patent). By doubling the number of diode arrays, the medium can be pumped more intensively and will produce a higher output. However, such a configuration requires correspondingly greater means for removing heat from the diodes and medium.
There remains a never-ending need for improved methods of removing the heat from the immediate vicinity of the diode pump sources and mediums in diode-pumped lasers, especially in side-pumped lasers, which can involve large numbers of diodes. There is also a constant need for other methods of increasing the power and/or efficiency of side-pumped lasers. Any device or arrangement which makes more efficient use of the excitation energy from the diode(s) in exciting the laser medium is desirable in every application; any configuration that allows a laser head having a limited size to put out a more powerful laser beam is also useful in any situation where higher power is desirable. Both factors are especially important for lasers that operate in a continuous wave mode, and for lasers used in devices where volume and weight are tightly constrained, such as in satellites.
One such improvement involves the use of reflective surfaces that are coated on or placed adjacent to one or more sides of a laser medium. Reflective surfaces are shown in various items of prior art, including U.S. Pat. No. 4,805,177 also assigned to applicant's assignee. In such devices, a beam of unfocused laser radiation from a laser diode array (or a bank of multiple diode arrays) enters the medium. Most of the energy in that beam of radiation is absorbed by the medium during the "first pass." However, depending on the size of the medium and the wavelength of the unfocused radiation, a portion of the radiation energy will pass entirely through the medium and reach the opposite side (i.e., the side away from the lasing diode pump sources). If there is no reflective surface on that side, the radiation will exit the medium; it will be lost, which reduces efficiency, and it can also impinge on other components of the laser head and contribute to the problem of heating. By providing a reflective surface (such as an electroplated gold or silver surface) adjacent to the medium on the side opposite the diode(s), some of the unfocused laser radiation energy which passes entirely through the medium during the first pass is reflected back into the medium with high efficiency, for a second pass. This reflected energy adds to the population inversion in the medium and hence increases the power and efficiency of the laser.
The reflective surface configuration shown in U.S. Pat. No. 4,805,177 is adapted for use with a laser medium excited by a single row of diode pump sources. By contrast, FIG. 6.59 of Koechner 1988 (cited above) on page 318, shows a side-pumped configuration involving four banks of laser diodes mounted at spaced locations in a hemispherical configuration around a single laser rod. The laser rod is mounted on a heat-conductive mounting device, in a rounded groove that holds the cylindrical medium. The groove is coated over a portion of its surface with a reflective surface as described in the prior paragraph. The configuration shown in that arrangement, however, requires that the banks of laser diodes be placed a substantial distance away from the medium, which in turn requires special lenses to be mounted between the diode pump sources and the medium, to direct the unfocused laser radiation from the diodes toward the medium. Koechner 1988, on page 317, specifically teaches that the use of lenses is advantageous compared to the use of direct pumping in several respects.
Despite that teaching, the Applicants have developed a side-pumped configuration which is superior to the side-pumped configuration shown in Koechner 1988. The configuration of the subject invention is far more compact and lightweight than the devices shown in Koechner 1988 and can generate a beam having substantially more useful output power, per unit of volume occupied by the laser head by mounting the diodes very close to the lasing medium. Such compactness and light weight is highly desirable in numerous applications, including use in satellites and in any other locations that involve miniaturized electronics. In addition, by eliminating the need for lenses, the subject invention provides several additional advantages, including: (1) it eliminates the high cost of lenses which must be precision-ground, carefully polished, and coated with special materials to reduce transmission losses; (2) it eliminates the power losses that occur whenever laser radiation passes through a lens; (3) it eliminates the time, expense, and tedium of carefully mounting and precisely aligning numerous small lenses within the laser head; (4) it increases the durability and longevity of the laser head, rendering it less susceptible to breakage or misalignment due to accidental shocks or mishandling; and (5) it provides a compact construction which enables continuous wave operation with high power outputs.
One object of the subject invention is to provide a side-pumped laser which provides relatively high usable energy output from a highly compact, relatively lightweight laser head.
Another object of this invention is to provide a highly efficient laser head.
Another object of this invention is to increase the efficiency of side-pumped laser heads that operate in a continuous wave mode.
Another object of the invention is to create a side-pumped laser which allows numerous banks of laser diode pump sources to excite a single laser medium, without requiring focusing lenses between the diodes and the medium.
Another object of this invention is to create a side-pumped laser with improved heat control and heat-removal means in a construction having more than one row of laser diode pump sources.
Another object of this invention is to create a side-pumped laser which uses more than one row of laser diodes to excite a laser medium, and which provides maximal utilization of the laser pump radiation by reflecting a substantial portion of the radiation energy which passes entirely through the medium back into the medium.
These and other objects and advantages of the present invention will become apparent after considering the following detailed specification which discloses preferred embodiments in conjunction with the accompanying drawings.